The detection of the origin of unfriendly fire has become increasingly important in modern highly mobile combat situations where a fluid combat zone cannot easily geographically distinguish between friendly and unfriendly fire. A number of efforts have been made in the art to make distinctions between such fires, and the projectiles resulting therefrom, but these efforts have, from a practical point of view, been unsuccessful until the advent of the invention described in U.S. Pat. No. 5,241,518, commonly assigned herewith. That patent describes a method and apparatus for determining the trajectory of a supersonic projectile of unknown velocity and direction. In the apparatus, at least three spaced-apart sensors, each having at least three transducers, are capable of encountering a shock wave generated by a supersonic projectile passing in the vicinity of the sensors and capable of generating signals in response to that shock wave. Those signals, it was found, are related to the azimuth and elevation angle of a unit sighting vector from each sensor to the origin of the shock wave. The apparatus provides means, e.g. a computer, for calculating from the signals the azimuth and elevation angle of the unit sighting vector from each sensor to the origin of the shock wave and for calculating from the unit sighting vectors of each of the three sensors the azimuth and elevation angle of the local trajectory of the projectile. Of course, the shock wave is propagated by the projectile, and by measuring the time lapses of the shock wave in passing the transducers of the sensors, it was found possible to calculate the trajectory of that supersonic projectile.
For the foregoing purposes, the sensors of that apparatus (including the transducers) are sensitive to the shock wave produced by the supersonic projectile and are, particularly, sensitive to the shock front and the ambient density lines thereof. From the shock wave and ambient density lines, the length of the projectile can be calculated from the time lapse of the passage of the shock wave and the ambient density line over a sensor. By thus knowing the length of the projectile, as well as the trajectory, from known look-up tables of the projectile length and the characteristics of the gun from which the projectile is fired, e.g. a cannon, the actual origin (the gun) of that projectile can also be calculated.
While the foregoing is a very brief summary of that U.S. patent, it will be appreciated that the apparatus and method of that patent are applicable only to determining the above-described characteristics of a supersonic projectile. Further, it will be appreciated that the apparatus and method involved, while quite applicable to modern battlefield conditions, are expensive and somewhat complex, which might not be applicable to other than battle conditions, e.g. not applicable to local insurrection, sniper fire, assassin fire and the like.
An effort to define the direction of a pressure wave in a more simple manner is reported in United Kingdom Patent No. GB 2 246 861 B, where four pressure sensitive transducers are spaced about the surface of a sphere in a tetrahedral configuration, i.e. the positions of the four transducers on the surface of the sphere form the apices of a tetrahedron. The size of the sphere is, of course, relatively critical, since the size must be small enough to avoid disrupting the pressure wave pattern, but on the other hand, large enough to provide a reasonable time delay between the pressure wave's impingement on the separated four transducers. Thus, the sphere must also not be too small. Time intervals between the arrival of the pressure wave to pass each transducer are measured, which provides the velocity of the pressure wave, and the three direction cosines of the normal to the pressure wave. When the orientation of the sphere to the pressure wave is known, the direction cosines of the axis to each of the four transducers can be calculated, since the angle subtended by the normal to the pressure wave and to the center of the sphere can be determined.
This device, however, has a number of serious drawbacks: the sphere configuration must be used, which would be quite conspicuous on, for example, the roof of an automobile; the sphere can be easily blinded to the pressure wave, e.g. the sphere is on a trunk of an automobile and the pressure wave comes from the front of the automobile; the sphere can be easily destroyed by sniper fire; and the sphere must be oriented to the pressure wave. Thus, this device is not practical for use in the environments intended by the present invention.
It would, therefore, be of substantial advantage to the art to provide methods and apparatus which are applicable to determining the general direction of the origin of a projectile from unfriendly fire in other than battle conditions, such as those noted above. Thus, such apparatus and methods could be utilized, for example, to locate unfriendly sniper fire in close quarters, such as in towns and cities, or, likewise, the fire of criminals or assassins in such circumstances. By knowing the general direction of the origin of a unfriendly fired projectile, the location of the criminal, sniper or assassin can be determined for return suppressing fire.